Abstract
Effect of molecular structure on the excited state dynamics, molecular rotation, and intramolecular charge distribution in the excited states of two structurally similar coumarin dyes, namely, coumarin 343 (C-343) and 7-diethyl amino coumarin 3-carboxylic acid (D-1421), on the Au nanoparticle (NP) surface has been investigated using steady state and time-resolved emission spectroscopy. In the first excited state (S1), both the coumarin dyes exist as a locally excited (LE) state in nonpolar solvent; however, in polar solvent, C-343 exists as intramolecular charge transfer (ICT) state and D-1421 predominantly exists as a twisted intramolecular charge transfer (TICT) state. Photoexcited C-343 molecules transfer energy to Au NP in aqueous solution; however, photoexcited D-1421 molecules do not transfer energy to Au NP due to poor overlap between emission band of D-1421 and plasmon absorption band of Au NP. Interestingly, emission intensity of the S1 state of D-1421 drastically increases on the Au NP surface due to restriction of amino-group rotation which is responsible for the population of TICT states. Intramolecular charge distribution in the excited states for both C-343 and D-1421 dyes found to be restricted on Au NP surface.